Berserker is Frankenstein's monster, the homunculus created by Victor Frankenstein appearing in Mary Shelley's novel. Into the Great Holy Grail War Berserker is summoned by Caules Forvedge Yggdmillennia with Victor Frankenstein's plans for a "perfect human" as a catalyst.

Mad Enhancement has robbed her of most of her linguistic ability, limiting her responses to head-shaking and grunts. She can differentiate friend and foe, and she is able to communicate simple ideas. Due to a lack of skill in expression, and fearful of her own monstrous nature, she maintains a shy silence that conceals her high intelligence and empathetic capabilities, except for the emotion of sorrow. She prefers to remain in material form, and she sullenly moans when her master forces her to remain in spiritual form. Rather than running wild, she prefers to pick flowers in the castle's courtyard garden or stare at the sky.

(Source: Type-Moon Wikia)